The present invention relates to an addressing system for a microprocessor and, more particularly, to an address modification system of instruction words, for example, to be applied to a communication protocol control LSI for a digital communication integrated network.
A digital communication integrated network, as described in "Microcomputer Handbook" p. 774 and p. 775, published in 25th Dec. 1985 by Ohmsha Co., Ltd., includes (1) an integrated service digital network (ISDN) where telephone, data, facsimile communication and various other sorts of communication processing services are integrated and provided in one digital communication network, and (2) an information network system (INS). In a switching station to which various sorts of terminal equipment is connected, a communication protocol control LSI for effecting communication processing between the station and prescribed terminal equipment is installed. In the communication protocol control LSI, when communication processing is started between the station and the prescribed terminal equipment, the function of accessing a data table is required for each terminal. Each data table should have data necessary for communication processing, such as data relating to sort of the terminal equipment or data related to communication speed. In the access of the communication protocol data table, since high speed processing is required, selection of the data may be effected through a branch instruction. In this case, a plurality of software steps such as loading of reference address data for varying the data table, adding processing of the displacement amount to the loaded reference and storing of the adding result are required.
However, in processing data through use of a branch instruction, high speed access of the communication protocol data table required in the communication protocol control LSI cannot be accomplished. Prior application U.S. Ser. No. 156,313 filed on Feb. 16, 1988, now U.S. Pat. No. 4,989,135, is related to a communication protocol control LSI.